The present application relates to fault detection in electrical power transmission and distribution systems. It finds particular application to the detection and analysis of faults in underground and other cables used in the transmission and distribution of electrical power.
Underground and other cables are a key component in the transmission and distribution of electrical power. Unfortunately, however, cables can be prone to shorts or otherwise abnormally low impedance connections between two or more phases or between one or more phases and ground. These and other cable faults can be caused by a number of factors, including human error (e.g., accidentally cutting or striking a cable), climatologic conditions (e.g., precipitation, seismic activity, or lightning strikes), animal activity, and failure or degradation of the cable or its associated equipment. Moreover, cable faults can lead to power outages, which are inconvenient for the affected customers and which can be expensive for the electric utility involved.
One category of fault is that of self-clearing faults. While self-clearing faults can have any number of root causes, they typically have a temporal duration which is insufficient to trip the relevant protective device. In practice, the duration of most self-clearing faults is typically less than about two (2) to three (3) cycles of the power system frequency, and in many cases less than one (1) cycle.
One mechanism which can generate self-clearing cable faults is a breakdown in the insulation between cable phases or between a cable phase and ground. Such faults are often caused or exacerbated by moisture at a cable splice or joint, and are typically characterized by an elevated or fault current having a duration of about one-quarter to one-half cycle (i.e., roughly four (4) to eight (8) milliseconds (mS) in a sixty Hertz (60 Hz) system). The onset of the fault current usually occurs at or near a voltage peak. As the situation deteriorates, the frequency and severity of these faults tend to worsen with time, culminating in an eventual cable failure and a resultant power outage.
As a consequence, a fault detection apparatus has been incorporated in a protective relay platform which can be used as an intelligent electronic device (IED). See Kojovic, et al., Sub-Cycle Overcurrent Protection for Self-Clearing Faults Due to Insulation Breakdown (1999); U.S. Pat. No. 6,198,401 to Newton, et al., Detection of Sub-Cycle, Self-Clearing Faults, issued Mar. 6, 2001. More particularly, and as more fully described in the references, the apparatus samples the cable current signal as it occurs. Contemporaneously with detecting a current signal which exceeds a threshold value, the apparatus confirms that the circuit breaker did not operate and also evaluates succeeding current samples (again, contemporaneously with their occurrence) to determine whether duration of the fault is less than one (1) cycle. If these conditions are satisfied, the device increments a fault counter. If the number and/or frequency of such faults occurrences exceeds a certain setting, the apparatus initiates an alarm, signalization, and/or a trip. In an alternate implementation, the apparatus also determines whether the fault occurred near a voltage peak.
However, the fault detection apparatus is provided at the level of the protective relay. One consequence of this relay-centric architecture is that the apparatus is relatively poorly integrated with the substation automation (SA), distribution automation (DA), feeder automation (FA), or other automation system. Moreover, the apparatus requires the use of a specialized hardware platform which must be closely coupled to the protective relay, and the detection techniques have been relatively unsophisticated.